How to lock the size when complete a video post?

In Katlura Plugin -
1) I click to add interactive video
2) I choose to post a video
3) I use webcam
4) I record my video
5) There are three choices to choose size(s) - Large or small or custom width.

Now that is one I need to know how to lock that custom width size (300) . That is what I want for all authors to use every time. Can you help me out? I am not expert on php code but you might need to write the steps for me to put the code in.

Thank you in advance if you can help.